This invention relates to a so-called glazed ceramic substrate, which is obtained by forming a glass coating layer on a ceramic substrate and is of use in electric or electronic devices.
Recently there has been an increasing trend to glaze a major surface of a ceramic plate, such as alumina plate, in order to provide a very smooth surface to the ceramic plate and utilize the glazed ceramic plate as a substrate of an electric or electronic device. A ceramic substrate having a glass coating is commonly called a glazed ceramic substrate. As to the material of the glass coating layer, it has been prevailing to use a sort of lead glass of which principal components are SiO.sub.2 and PbO. Particularly there is an increasing trend of using glazed ceramic substrates for maufacturing hybrid integrated circuits, thermal heads of thermal printers or facsimile receivers.
In the recent thermal printers including those installed on the printing devices in facsimiles, still there is a demand for improvements in both the resolving power and the picture-producing speed of the thermal heads. To meet such a demand there is the need of further reducing the size of each heating element in the thermal heads and, besides, raising the heating temperature. Such modifications in the fundamentals of a thermal head inevitably result in an increase in the amount of heat generated per unit area of the substrate. In the case of a glazed ceramic substrate, therefore, an enhanced stability at high temperatures is required to the glass forming the coating layer. However, the aforementioned SiO.sub.2 -PbO base glass is insufficient in its high temperature stabiity when evaluated as the coating material of the substrate of a high-performance thermal head. In practice, the insufficiency of high temperature stability of the glass takes shape in softening, cracking or crystallization of the glass coating layer.
The glass coating layer of a glazed ceramic substrate is required to be high also in resistance to chemicals, because the interconnections between the circuit elements to be mounted on the substrate are formed by partly removing a thin film of metal deposited on the surface of the substrate by chemical etching after the formation of a patterned film of photoresist as a mask so as to leave desirably patterned conduction paths. However, the aforementioned lead glass is not fully satisfactory in its resistance to chemicals.